This invention relates to a period and frequency measuring instrument which measures period and frequency of a relatively low frequency input signal with high accuracy.
In measuring frequency or period of an input signal of relatively low frequency by a period and frequency measuring instrument, the input signal is used as a gate signal for gating internal clock signals to a counter which counts the number of clock pulses to determine the period and thus the frequency of the input signal.
FIG. 1 shows one of the conventional period and frequency measuring instruments of this kind. A division factor M of a divider 102 which divides the number of pulses in an input signal into 1/M pulses is selected corresponding to the frequency of the input signal. FIG. 2A shows that an input signal 201 that is to be measured is supplied at an input terminal 101 and provided to the divider 102 whose output generates the divided output 203 shown in FIG. 2C. The divided output 203 is transmitted to a gate signal generator 103 comprising, for example, a toggle-type flip-flop. The gate signal generator 103 generates the gate signal 204 shown in FIG. 2D which has the length of one period of the divided input signal 203 derived from the divider 102. The gate signal 204 thus generated is supplied to an AND gate 104 to open the AND gate 104 with its high level.
The other input terminal of the AND gate 104 is provided with a clock signal 205 having a constant period T.sub.0 from a clock signal generator 105 as shown in FIG. 2E. When the gate signal 204 is at high level, the clock signal 205 is supplied via the AND gate 104 to a counter 106. FIG. 2F shows the gated clock signal 205 derived from the AND gate 104. The total number of the gated clock pulses N is counted by the counter 106.
The counted result is provided to a calculator 107 wherein the frequency F and the period P of the input signal 201 are calculated. Namely, in the case that the number of the clock signals 205 counted by the counter 106 is N, the input frequency and period are obtained by calculating, F=M/NT.sub.0 and P=NT.sub.0 /M, respectively. The calculated frequency or period is displayed by a display 108. The operation described above starts after a reset signal 202 provided from a terminal 221 as shown in FIG. 2B.
In this conventional period and frequency measuring instrument, there arise fractional times .DELTA.T.sub.x1 and .DELTA.T.sub.x2 corresponding to the time differences between the rise of the gate signal 204 and the first gated clock signal and between the fall of the gate signal 204 and the immediately following clock signal as illustrated in FIG. 2F. In order to make the fractional times .DELTA.T.sub.x1, .DELTA.T.sub.x2 small so as to improve measurement accuracy, it is necessary to use clock signals of higher frequency. However, utilization of the higher frequency clock signals requires both the AND gate and the counter 106 to operate at high speed, which makes the instrument expensive. Although the fractional times .DELTA.T.sub.x1, .DELTA.T.sub.x2 represent information effecting the accuracy of the calculated period or frequency of the input signal, conventional instruments are not able to take these fractional times into account.